This proposal requests support for a Keystone Symposia meeting entitled Mucosal Biology: A Fine Balance between Tolerance and Immunity, organized by Joanne L. Viney, Paul Garside and Bana Jabri, which will be held in Vancouver, British Columbia, Canada from February 26 - March 3, 2011. Experts from the basic science and clinical fields of mucosal biology will be brought together with leaders from other scientific disciplines for a stimulating discussion on the mechanisms regulating the maturation, development and maintenance of mucosal responses in health and disease. Attention will be given to gut, lung, skin, eye and genitourinary tract tissues. The latest information on novel cell types and receptors important for regulating immunity and enhancing mucosal protection will be presented. The influence of infection on driving altered host immunity will be interrogated. The meeting will highlight modern approaches to vaccine development and imaging technology, and the most up to date advances from the physical sciences and engineering will be discussed as we look to the medical needs and developments of the future. In addition, opportunities for interdisciplinary interactions will be significantly enhanced by the concurrent meeting on Immunity in the Respiratory Tract: Challenges of the Lung Environment, which will share a keynote address and two plenary sessions with this meeting. Project Narrative: The mucosal surfaces of the body must mount protective immune responses against potential pathogens and, at the same time, remain tolerant to beneficial antigens, such as those in food proteins. On occasion, the balance between tolerance and immunity is upset, resulting in chronic inflammatory disorders such as inflammatory bowel disease (IBD) in the intestine, and chronic obstructive pulmonary disease (COPD) in the lung. The regulatory mechanisms that control whether, and how, host cells mount an active immune response remain to be fully elucidated, as do the regulatory mechanisms that help to avoid autoimmune inflammatory responses.